Tertiary structures of proteins can be very complex. Analyzing and recognizing common structural patterns is the first step in understanding how proteins form higherâorder structures. Two terms, domain and motif (also called fold), describe protein structural patterns. Classify each characteristic as a domain or a motif.

a. Always independently stable

b. Retains native structure even when separated from the rest of the protein

c. β Barrel

d. Calcium-binding segments of calmodulin

e. An advantageous folding pattern composed of two or more secondary structure elements

f. Sometimes independently stable

a. Always independently stable. Domain

b. Retains native structure even when separated from the rest of the protein. Domain

c. β Barrel. Motif

d. Calcium-binding segments of calmodulin. Motif

e. An advantageous folding pattern composed of two or more secondary structure elements. Domain

f. Sometimes independently stable. Motif

Explanation:

a. Protein domains are evolutionary conserved and stable 3D structures formed by a series of amino acids joined by peptide bonds.

b. Domains are stable because they conserve their tridimensional structure in physiological conditions even if they are separated of the rest of the protein.

c. The ß-barrel motif is a type of motif localized in single-stranded DNA viruses.

d. Calmodulin is a protein that contains calcium-binding motifs that enable to sense intracellular calcium levels.

e. Domain folding patterns are critical transitional states that allow the proteins their interaction with specific ligands in different physiological conditions.

f. Motifs are sometimes independently stable, but due to their short length (3-15 amino acids), protein motifs are generally unstable in different conditions.